Perpendicular magnetic recording systems have been developed for use in computer hard disc drives to provide higher liner density than longitudinal recording. FIG. 1, obtained from Magnetic Disk Drive Technology by Kanu G. Ashar, 322 (1997), shows magnetic bits and transitions in longitudinal and perpendicular recording. In a longitudinal recording there exists a demagnetization field between two magnetic bits. These demagnetization fields tend to separate bits, making transition space between bits, that is, transition parameter a, large as shown in FIG. 1(a). At very high bit densities, the limiting parameter may be the length of the transition region. Perpendicular recording bits do not face each other, and hence they can be written at closed distances as shown in FIG. 1(b).
A typical perpendicular recording head includes a trailing read/write pole, a leading return or opposing pole magnetically coupled to the read/write pole, and an electrically conductive magnetizing coil surrounding the yoke of the write pole as shown in FIG. 2, Magnetic Disk Drive Technology by Kanu G. Ashar, 323 (1997). Perpendicular recording media may include magnetic media and an underlayer as shown in FIG. 2. The magnetic media could be a hard magnetic recording layer with vertically oriented magnetic domains and the underlayer could be a soft magnetic underlayer to enhance the recording head fields and provide a flux path from the trailing write pole to the leading or opposing pole of the writer. The magnetic flux passes from the write pole tip, through the hard magnetic recording track, into the soft underlayer (SUL), and across to the opposing pole. Such perpendicular recording media may also include a thin interlayer between the hard recording layer and the soft underlayer to prevent exchange coupling between the hard and soft layers. The soft underlayer helps also during the read operation. During the read back process, the soft underlayer produces the image of magnetic charges in the magnetically hard layer, effectively increasing the magnetic flux coming from the medium. This provides a higher playback signal.
The soft underlayer is located below a recording layer and forms a mirror image of the recording head. Together with the image head, there are essentially two heads involved in each recording event; thus, the net recording field becomes fairly large compared to the field generated with a longitudinal head. Magnetic flux flows from head through the SUL to return pole crossing twice through the recording layer. The return pole is generally much wider than the writing pole in order to dilute the magnetic flux intensity flow back through the recording layer. In spite of this, it is sometimes found that writing also occurs at the return pole, with the consequence that data can be partially erased not only on the track being recorded, but also on an adjacent track resulting in unintentional erasure of data stored in the recording layer. The quality of the image, and therefore the effectiveness of the soft underlayer, and erasure of data both depend on the permeability of the soft underlayer. Thus, there is a need for a perpendicular recording medium having a soft underlayer that forms a good mirror image of the recording head without erasure of data in the recording layer.